A modulator system

ABSTRACT

The present invention relates to a modulator system adapted to generate high voltage pulses suitable for supply across a high voltage load having a thermionic cathode, such as a magnetron. The modulator system comprises a high voltage DC PSU connected to a switching mechanism adapted to generate high voltage pulses from the high voltage DC PSU for application to a thermionic cathode of a high voltage load. The modulator system further comprises an isolation transformer; a heater PSU adapted to be connected to a cathode heater through the isolation transformer and to provide an AC current thereto. The modulator system further comprises a controller to receive pulse instruction signals and trigger generation of corresponding high voltage pulses by the switching mechanism, to calculate the estimated arrival time of a next pulse instruction signal, based on the time between previous pulse instruction signals, and disable the heater PSU for a preset time, commencing before the estimated arrival time of the next pulse instruction signal, such that no current is supplied from the heater PSU while current is supplied from the high voltage PSU.

This invention relates to a modulator system adapted to generate highvoltage pulses suitable for supply across a high voltage load having athermionic cathode. Such a load may be, for example, a magnetron.

BACKGROUND

Modulators may be used to control the generation of high voltage pulsesfor supply across a load, for example a magnetron. In the case of amagnetron load, such modulators will have components i.e. those directlyconnected to magnetron, at very high potentials, which must be isolatedfrom components at lower potentials. For example, it is conventional tooperate magnetrons with the main body forming the anode at earthpotential and with the cathode and cathode heater to be at a highnegative potential. This requires the cathode heater to be powered via ahigh voltage isolation barrier.

Such modulators and magnetrons may be used in linear acceleratorsystems, known as linacs, for x-ray generation. Such linacs can be usedin medical applications, such as radiotherapy systems, and industrialapplication such as cargo scanning. In medical applications, veryaccurate control of the output of the magnetron is required as theoutput will affect the final dose received by the patient.

Some cathode heaters are operated at mains frequency via a mainstransformer that has the required high voltage isolation. In such asystem the operating frequency of the magnetron will vary as the ACheater current causes a varying magnetic field in the heater coil thatwill interact with the magnetic field of the magnetron magnet. Also, thelow frequency of mains distribution can mean that AC heating can causeundesirable resonance within the magnetron structure leading to failure.On systems that require frequency stability throughout the magnetronpulse, for example in medical applications, it is conventional torectify and smooth the isolated AC supply to the cathode heater. This isnormally done at high frequency, normally greater than 100 kHz, toreduce the capacitance and therefore the size of the capacitors requiredto reduce the heater supply ripple to an acceptable level. Other cathodeheaters use DC heating so as to avoid the issues associated with ACheating.

BRIEF SUMMARY OF THE DISCLOSURE

In accordance with the present disclosure, there is provided a modulatorsystem adapted to generate high voltage pulses suitable for supplyacross a high voltage load having a thermionic cathode, the modulatorsystem comprising: a high voltage DC PSU; a switching mechanismconnected to the high voltage DC PSU and adapted to generate highvoltage pulses from the high voltage PSU for application to a thermioniccathode of a high voltage load; an isolation transformer; a heater PSUadapted to be connected to a cathode heater through the isolationtransformer and to provide an AC current to the cathode heater, thecathode heater being suitable for use with the thermionic cathode; and acontroller adapted to receive pulse instruction signals and triggergeneration of corresponding high voltage pulses by the switchingmechanism, calculate the estimated arrival time of a next pulseinstruction signal, based on the time between previous pulse instructionsignals, and disable the heater PSU for a preset time, commencing beforethe estimated arrival time of the next pulse instruction signal, suchthat no current is supplied from the heater PSU while current issupplied from the high voltage PSU.

Preferably, the AC current has a frequency in the range 10 Hz to 50 kHz.

Preferably, the heater PSU generates the AC current from a DC supplyusing a DC-AC converter.

Preferably, the isolation transformer provides isolation in the range of20 kV to 80 kV. For example, isolation of 65 kV may be provided.

Preferably, the pulse instruction signals have a non-regular frequency.

Preferably, the pulse instruction signals have a frequency in the range6 Hz to 1 kHz.

Preferably, the switching mechanism of the modulator system is asolid-state switching mechanism.

In accordance with the a further aspect of the present disclosure, thereis provided a high voltage arrangement comprising a modulator system asclaimed in any preceding claim and a high voltage load connected to themodulator system.

Preferably, the high voltage load of the high voltage arrangement is amagnetron.

In accordance with another aspect of the present disclosure, there isprovided a linear accelerator system comprising a linear accelerator anda modulator system as described herein.

In accordance with the present disclosure, there is further provided amethod of generating high voltage pulses suitable for supply across ahigh voltage load having a thermionic cathode, the method operating in amodulator system comprising a high voltage pulse DC PSU adapted to beconnected to a thermionic cathode, a heater PSU adapted to be connectedto a cathode heater through an isolation transformer , the cathodeheater being suitable for use with the thermionic cathode; and acontroller, the method comprising the heater PSU providing an AC currentto the cathode heater; the controller receiving pulse instructionsignals, each comprising instructions in relation to a requested highvoltage pulse, calculating the estimated arrival time of a next pulseinstruction signal, based on the time between previous pulse instructionsignals; for a particular requested high voltage pulse, disabling theheater PSU for a preset time longer than that of the requested highvoltage pulse, commencing before the estimated arrival time of the nextpulse instruction signal; while the heater PSU is disabled, triggeringgeneration of the requested high voltage pulse, corresponding to thepulse instruction signal, from the high voltage PSU.

Preferably, the method comprises the heater PSU providing an AC currentto the cathode heater at a frequency of in the range 10 Hz to 50 kHz.

Preferably, the method comprises the heater PSU generating the ACcurrent from a DC supply using a DC-AC converter.

Preferably, the method comprises receiving pulse instruction signals ata non-regular frequency.

Preferably, the method comprises receiving pulse instruction signals ata frequency in the range 6 Hz to 1 kHz.

Preferably, the high voltage load is a magnetron.

In accordance with another aspect of the present disclosure, there isprovided a isolation transformer suitable for use in a modulator forgenerating high voltage pulses for supply across a high voltage loadhaving a thermionic cathode, the isolation transformer comprising aprimary winding formed from a triaxial cable where the triaxial cablecomprises a core conductor surrounded by a dielectric insulator, in turnsurrounded by a screening conductor, with an outer insulating jacket.

Preferably, the screening conductor is formed from mesh braid.

Preferably, the screening conductor is formed from two layers of closemesh braid.

Preferably, each layer has greater than 80% coverage.

Preferably, each layer of braid is connected to a safety earth.

Preferably, the two layers of braid in the screen conductor are indirect contact with each other.

Preferably, the isolation transformer comprises a ferrite core.

Preferably, the core is formed of high-frequency Nickel-Zinc ferrite.

Preferably, the core is formed of a ferrite having at least one of: apermeability of greater than 1500, a saturation flux density of greaterthan 3000 Gauss; and a volumetric resistivity of between 2.5×10⁹ Ohm cmand 0.5×10⁸ Ohm cm. For the example, the ferrite may provide a minimumleakage of 10 μA at 25 kV and a maximum earth leakage of 1.3 mA at 65kV.

Preferably, the isolation transformer comprises a core of material typeCMD5005, CMD908 or N16.

Preferably, the isolation transformer comprises a toroidal core.

Preferably, the transformer is an oil-filled transformer.

Preferably, the transformer is closely coupled.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a modulator system according to thedisclosure;

FIG. 2 is a representation of a portion of the isolation transformershown in FIG. 1;

FIG. 3 is a representation of a cutaway view of a triaxial cable used inthe isolation transformer shown in FIGS. 1 and 2;

FIG. 4 is a flowchart of a mode of operation of the modulator system;

FIG. 5 is a timing diagram in relation to the operation of the modulatorsystem.

DETAILED DESCRIPTION

Referring initially to FIG. 1, there is a shown a modulator system,indicated generally by the reference numeral 100, connected to a highvoltage load having a thermionic cathode, in this case a magnetron 102.The modulator system is adapted to generate high voltages pulse suitablefor supply across the load. The modulator system comprises a controller104, a heater Power Supply Unit (PSU) 106, an isolation transformer 108,a high voltage Power Supply Unit (PSU) 110 and a switching network 112.

Together, the switching mechanism 122 and the isolation transformer 108may be considered as a modulation unit 125, the output of which issupplied to the high voltage load. The modulation unit 125 is indicatedin the FIG. 1 by a box of dotted lines.

The magnetron comprises a magnetron cathode 114, a magnetron anode 115and a cathode heater 116. The magnetron 102 is arranged in theconventional configuration with its anode 115 at ground potential. Themagnetron cathode 114 is connected to a negative high voltage withrespect to ground by the high voltage PSU 110 and switching network 112.The cathode heater 116 has a first terminal 118, connected to thecathode 114; and a second terminal 117 being connectable to the heaterPSU 106 via a heater terminal (not shown) of the magnetron.

The isolation transformer comprises a primary winding 119; a core 120,preferably a toroidal ferrite core; and a secondary winding 122. Thewindings 119, 122 and core 120 may be encapsulated and/or immersed in asuitable transformer oil. The isolation transformer 108 is preferablyphysically compact, which is enabled by the choice of core shape andmaterial and by the use of encapsulation in oil.

The magnetron heater terminals 117, 118 are connected to the secondarywinding 122 of the isolation transformer 108. This provides a highvoltage insulation barrier between the cathode voltage and the heaterpower supply 106 which is connected to the primary winding 119 of theisolation transformer 108. In an embodiment, the isolation transformeris able to provide isolation up to −65 kV, however in other embodiments,higher isolation values are available.

The controller 104 receives pulse instruction signals 124 from anexternal source. The controller provides processing and timingfunctionality. In the illustrated arrangement, these functionalities areprovided by a suitable FPGA 105 a and DSP 105 b. The pulse instructionsignals 124 may be considered asynchronous in relation to the operationof the modulator system 100, as they do not necessarily occur at fixedor known time intervals or according to a known schedule or timetable.The pulse instructions signals 124 may therefore be considered as havinga non-regular frequency. The pulse instruction signals 124 define arequested output to be produced by the load e.g. a pulse of RF energy tobe emitted by the magnetron 102. The pulse instruction signals 124 mayspecify the characteristics of the requested output, for example, in thecase of the magnetron 102, the pulse instruction signals 124 may specifythe repetition rate of the RF energy pulse to be emitted and theduration of the RF pulse to be emitted. The controller 104 processes thepulse instruction signals 124 and sends control signals to the othercomponents of the modulator 100 so as to deliver the requested output.In the case of a magnetron load, the output includes one or moresuitable voltage pulses supplied to the magnetron 102 so as to provide apulse of RF energy. The controller 104 is connected to the heater PSU soas to provide an enable/disable instruction and instructions relating toheater power settings.

When the controller is providing a heater PSU enable signal, the heaterPSU 106 provides AC current to the cathode heater via the isolationtransformer 108. The heater PSU receives a DC input from which the ACcurrent is generated using a DC to AC converter. The frequency of thecurrent supplied to the cathode heater is in the range of 10 kilohertzto 50 kilohertz. In the absence of the enable signal, the heater PSU isin a shut-down state and not providing current. The enable/disableinstruction provided by the controller 104 to the heater PSU 106 may bein the form of a single signal where a ‘high’ value acts as a disablesignal and a ‘low’ value acts as an enable signal, or vice versa, or byother implementation, for example, where the enable/disable function isachieved via multiple signals.

The switching mechanism 112 may be implemented using solid statedevices, for example a stack of FET or IGBT switch modules and suitableaccompanying capacitors. The switching mechanism 112 may correspondsubstantially to that described in International Patent ApplicationPublication No. WO 2002/104076, International Patent ApplicationPublication No. WO 2012/001409 and related documents. The switchingmechanism 112 will not be described in further detail here. Theswitching mechanism could also be implemented using other techniquessuch as thyratrons, inline type modulators, and so on.

Referring now to FIG. 2, there is shown a representation of a portion ofthe isolation transformer, comprising the toroidal core 120, the primarywinding 119 and secondary winding 122. The primary winding 119 is formedfrom a triaxial cable 200, shown in an illustrative cutaway view in FIG.3, comprising a core conductor 204 of 50/0.25 tinned annealed copperwire surrounded by a dielectric insulator 206 of FEP to diameter 2.60mm, which in turn is surrounded by a screening conductor formed from afirst layer of close mesh braid 208 and a second layer of close meshbraid 210. In this embodiment, the close mesh braids are identical inmaterial, with the outer layer being of larger diameter. The close meshbraids 208 and 210 may be 0.1 mm diameter tinned annealed copper with84% nominal coverage. In another embodiment, the screening conductorcould be formed by a metal foil but mesh braids are capable of carryingmore current. A single mesh braid could be used instead of two but itscoverage must then be greater than would be required with a two-layerconfiguration. The dielectric insulator 206 insulates the core conductor204 from the screening conductor. The inner layers 204, 206, 208, 210 ofthe triaxial cable are surrounded by an outer insulating jacket 220,which may be formed from a polymer or other insulating substance. Eachof the two layers 208, 210 in the screening conductor are connected to asafety earth 202.

The core conductor 204 forms the current path of the primary winding 119of the isolation transformer 108, being connected across the outputterminals of the heater PSU 106.

The core 120 of the transformer is made from a high-frequencynickel-zinc ferrite, having a high permeability, high resistivity,narrow BH loop and closed porosity. Typical parameters of this preferredferrite include an initial permeability of 2100, a maximum permeabilityof 5500, a saturation flux density of 3300 Gauss, a remanent fluxdensity 1300 Gauss, a coercive force of 0.12 Oersted, a Curietemperature of 130° C., dc volume resistivity of 10¹⁰ ohm-cm and a bulkdensity of 5.27 g/cc. A suitable ferrite for use in the core 120 may beobtained from Ceramic Magnetics, Inc. under material number CMD5005.Other possible materials are CMD908 and N16. Preferably, the core 120 istoroidal but other core shapes may be used.

The core may have at least one of: a permeability of greater than 1500,a saturation flux density of greater than 3000 Gauss; and a volumetricresistivity of between 2.5×10⁹ Ohm cm and 0.5×10⁸ Ohm cm, giving aminimum leakage of 10 μA at 25 kV and a maximum earth leakage of 1.3 mAat 65 kV.

During operation of the modulator, high voltage pulses on the secondarywinding 120 from high voltage power supply 110 can become coupled to theprimary winding 119 by stray capacitance. The earthed layers 208, 210 ofscreening conductors form an electrostatic shield to decouple the coreconductor 204 from any such stray capacitance and thereby any influenceof the high voltage pulse from high voltage power supply 110 andswitching network 112.

In the event of a failure of the insulation in the isolationtransformer, an arc may form between the secondary winding and thescreening conductor, resulting in a reduction in or absence of theisolation provided by the transformer. However, by connecting each layerof the screening conductor to the safety earth 202, a safe path toground for any high voltage energy is provided. In this way, thepotential of core conductor 204 will not be affected by any high voltagearc, thus ensuring that any components or circuits connected to the coreconductor 204 of the primary winding 119 are not exposed to highvoltages. In this way, the integrity of the isolation may be maintained.Having a double layer screening conductor ensures that this protectionis provided even if there is a defect in one of the layer of the screenconductor and ensures a coverage of close to 100%, each individualscreen having an 84% coverage. The primary winding could instead beformed by a co-axial with a solid screen, but this would be moredifficult to wind.

The increased safety provided by the connection of the screeningconductor to ground allows for reduction in the proving testing requiredfor the isolation transformer.

The use of a screened core conductor in the primary winding allows forrelatively simple, and consequently less expensive, transformerconstruction. Creation of the primary winding is simplified by the useof the screen core conductor.

Referring now to FIGS. 4 and 5, a method of operation of the modulatoris described. In step 400, the controller 104 measures the time intervalt_(m1) between the arrival of consecutive pulse instruction signals, forexample pulse instruction signals I₁ and I₂. In the step 402, thecontroller predicts when the next pulse instruction signal 124 willarrive, based on the measured interval tml between previous pulseinstruction signals. The estimated time until the next pulse instructionsignal t_(e1) is shown between pulse instructions signals I₂ and I₃ inFIG. 5. In one example, the controller predicts that the estimated timeuntil the next pulse instruction signal t_(e) will be the same as theinterval between previous pulses. In other examples, t_(e) may be basedon an average of previously measured interval times or othercalculations.

In step 404, in advance of the next predicted pulse instruction signal,the controller 104 sends a disable signal to the heater PSU 106, asshown by pulses H in the heater PSU power disable trace shown in FIG. 5.In this way, the controller instructs the heater PSU 106 to stopsupplying power to the heater. In step 406, the controller receives theexpected pulse instruction signal I₃. In step 408, the controller 104sends the necessary instructions to the high voltage DC PSU 110 andswitching mechanism 112 to generate the output requested in the pulseinstruction signal. Next, in step 410, when the requested high voltagepulse has been generated by the switching mechanism, resulting in asuitable output of RF energy from the magnetron, the controller removesthe disable signal to the heater PSU 106 such that it is once moreenabled and recommences supplying power the magnetron heater.

In this way, by cutting off the power to the magnetron heater duringgeneration of the magnetron output, the method ensures that no currentis supplied from the heater PSU while current is supplied from the highvoltage PSU.

In this way, any interaction between the heater current and the magneticfield of the magnetron 102 is prevented. Such an interaction couldresult in frequency modulation of the output of the magnetron 102 andother undesirable effects. Frequency modulation of the output of themagnetron 102 may be unacceptable in certain applications, for examplemedical applications. The modulator disclosed herein allows for veryaccurate control of the output pulses of the magnetron.

By ensuring that the heater PSU is already disabled before the pulseinstruction signal is received, the modulator is highly responsive tothe pulse instruction signal. Any delay that would result from acting todisable the heater PSU only after receipt of the pulse instructionsignal is eliminated. Furthermore, the modulator is adaptable to a widerange of output pulse repetition rates including non-uniform outputpulse rates. The modulator is capable of operating in a one-shot mannerto generate a single demanded output pulse, and is also capable ofoperating to provide output pulses at a rate of 6 per second up to 1000per second.

Referring again to FIG. 5, in relation to pulse instruction signal I₄,it can been seen that this pulse instructions signal was expected attime t_(e2) after the previous pulse, where t_(e2) is equal to t_(m2),the time between pulses I₂ and I₃. As can be seen, the period duringwhich the heater PSU is disabled at H₂ is greater than the precedingperiod at H₁ to take account of the estimated arrival time of the nextpulse instruction signal and the actual later arrival time t_(m3), asshown at I₄. The subsequent pulse instruction signal I₅ then arrives atthe same time interval as the preceding. The repetition frequency isthus lower compared to that of the first three pulse instruction signalsand the change in the repetition frequency occurs during a period whenthe heater PSU is disabled.

The use of AC heating with a frequency in the range 10 kHz to 50 kHzallows for a simple implementation of heater power supply and isolationtransformer. There is no smoothing or rectification required and nonegative EMC issues. This results in lower manufacturing costs. Thesystem is scalable to any heater power requirement simply by choosingthe appropriate transformer.

The modulator system described herein has been described in relation touse with a magnetron, however, it is also suitable for other highvoltage loads, for example a klystron.

The modulator system described herein may be used in a linearaccelerator system, in combination with a linear accelerator.

The present invention relates to a modulator system adapted to generatehigh voltage pulses suitable for supply across a high voltage loadhaving a thermionic cathode, such as a magnetron. The modulator systemcomprises a high voltage DC PSU connected to a switching mechanismadapted to generate high voltage pulses from the high voltage DC PSU forapplication to a thermionic cathode of a high voltage load. Themodulator system further comprises an isolation transformer; a heaterPSU adapted to be connected to a cathode heater through the isolationtransformer and to provide an AC current thereto. The modulator systemfurther comprises a controller to receive pulse instruction signals andtrigger generation of corresponding high voltage pulses by the switchingmechanism, to calculate the estimated arrival time of a next pulseinstruction signal, based on the time between previous pulse instructionsignals, and disable the heater PSU for a preset time, commencing beforethe estimated arrival time of the next pulse instruction signal, suchthat no current is supplied from the heater PSU while current issupplied from the high voltage PSU.

The present disclosure further relates to an isolation transformersuitable for use in a modulator for generating high voltage pulses forsupply across a high voltage load having a thermionic cathode such as amagnetron, the isolation transformer comprising a primary winding formedfrom a triaxial cable where the triaxial cable comprises a coreconductor surrounded by a dielectric insulator, in turn surrounded by ascreening conductor, with an outer insulating jacket.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of them mean “including but notlimited to”, and they are not intended to (and do not) exclude othermoieties, additives, components, integers or steps. Throughout thedescription and claims of this specification, the singular encompassesthe plural unless the context otherwise requires. In particular, wherethe indefinite article is used, the specification is to be understood ascontemplating plurality as well as singularity, unless the contextrequires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith. All of the features disclosed in this specification(including any accompanying claims, abstract and drawings), and/or allof the steps of any method or process so disclosed, may be combined inany combination, except combinations where at least some of suchfeatures and/or steps are mutually exclusive. The invention is notrestricted to the details of any foregoing embodiments. The inventionextends to any novel one, or any novel combination, of the featuresdisclosed in this specification (including any accompanying claims,abstract and drawings), or to any novel one, or any novel combination,of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

1. A modulator system adapted to generate high voltage pulses suitablefor supply across a high voltage load having a thermionic cathode, themodulator system comprising: a high voltage DC PSU; a switchingmechanism connected to the high voltage DC PSU and adapted to generatehigh voltage pulses from the high voltage PSU for application to athermionic cathode of a high voltage load; an isolation transformer; aheater PSU adapted to be connected to a cathode heater through theisolation transformer and to provide an AC current to the cathodeheater, the cathode heater being suitable for use with the thermioniccathode; and a controller adapted to receive pulse instruction signalsand trigger generation of corresponding high voltage pulses by theswitching mechanism, calculate the estimated arrival time of a nextpulse instruction signal, based on the time between previous pulseinstruction signals, and disable the heater PSU for a preset time,commencing before the estimated arrival time of the next pulseinstruction signal, such that no current is supplied from the heater PSUwhile current is supplied from the high voltage PSU.
 2. The modulatorsystem as claimed in claim 1 wherein the AC current has a frequency inthe range 10 Hz to 50 kHz.
 3. The modulator system as claimed in claim 1wherein the heater PSU generates the AC current from a DC supply using aDC-AC converter.
 4. The modulator system as claimed in claim 1 whereinthe isolation transformer provides isolation in the range of 20 kV to 80kV.
 5. The modulator system as claimed in claim 1 wherein the pulseinstruction signals have a non-regular frequency.
 6. The modulatorsystem as claimed in claim 1 wherein the pulse instruction signals havea frequency in the range 6 Hz to 1 kHz.
 7. The modulator system asclaimed in claim 1 wherein the switching mechanism is a solid-stateswitching mechanism.
 8. The modulator system as claimed in claim 1wherein the high voltage load is a magnetron.
 9. A high voltagearrangement comprising the modulator system as claimed in claim 1 and ahigh voltage load connected to the modulator system.
 10. The highvoltage arrangement as claimed in claim 9 wherein the high voltage loadis a magnetron.
 11. A linear accelerator system comprising a linearaccelerator and the modulator system as claimed in claim
 1. 12. A methodof generating high voltage pulses suitable for supply across a highvoltage load having a thermionic cathode, the method operating in amodulator system comprising a high voltage pulse DC PSU adapted to beconnected to a thermionic cathode, a heater PSU adapted to be connectedto a cathode heater through an isolation transformer , the cathodeheater being suitable for use with the thermionic cathode; and acontroller, the method comprising the heater PSU providing an AC currentto the cathode heater; the controller receiving pulse instructionsignals, each comprising instructions in relation to a requested highvoltage pulse, calculating the estimated arrival time of a next pulseinstruction signal, based on the time between previous pulse instructionsignals; for a particular requested high voltage pulse, disabling theheater PSU for a preset time longer than that of the requested highvoltage pulse, commencing before the estimated arrival time of the nextpulse instruction signal; while the heater PSU is disabled, triggeringgeneration of the requested high voltage pulse, corresponding to thepulse instruction signal, from the high voltage PSU.
 13. The method asclaimed in claim 12 comprising the heater PSU providing an AC current tothe cathode heater at a frequency of in the range 10 Hz to 50 kHz. 14.The method as claimed in claim 12 comprising the heater PSU generatingthe AC current from a DC supply using a DC-AC converter.
 15. The methodas claimed in claim 12 comprising receiving pulse instruction signals ata non-regular frequency.
 16. The method as claimed in claim 12comprising receiving pulse instruction signals at a frequency in therange 6 Hz to 1 kHz.
 17. The method as claimed in claim 12 wherein thehigh voltage load is a magnetron.